Is 3% a code requirement?

It appears as a recommendation in an informational note rather than a mandatory rule. Local amendments and project specifications frequently make it mandatory, so check the edition enforced in your area.

Does voltage drop apply to feeders too?

Yes. The informational note to NEC 215.2(A)(1) recommends no more than 3% on the feeder itself, and no more than 5% for the feeder and branch circuit combined. The 3% figure is not only a branch-circuit number.

Why does the calculator ask for one-way length?

The formula already doubles the distance for a single-phase circuit. Enter the distance from the source to the load, not the round trip.

Guide

The 3% Voltage Drop Rule, Explained

The 3% figure is a recommendation in an informational note, not a hard rule — but it is the number inspectors and equipment manufacturers expect you to design to.

Where the numbers come from

The recommendation is that a branch circuit not exceed 3% and that a feeder not exceed 3%, and that the total from the service to the last outlet not exceed 5%.

Because it sits in an informational note, it is not enforced the way a mandatory rule is. In practice it still matters: motors overheat, electronics misbehave and equipment fails to start on long, undersized runs.

How to compute it

Voltage drop depends on current, one-way distance, conductor size and material. For a single-phase circuit the drop is twice the one-way run because the current travels out and back.

Single-phase: VD = 2 x K x I x L / CM
Three-phase: VD = 1.732 x K x I x L / CM
K is about 12.9 for copper and 21.2 for aluminum
CM is the conductor circular-mil area

What to do when you exceed 3%

You have three practical levers, in order of cost:

Go up one or two conductor sizes (cheapest per unit, most common fix).
Shorten the run (rarely possible after design).
Reduce the load on that circuit.

Frequently asked

Is 3% a code requirement?: It appears as a recommendation in an informational note rather than a mandatory rule. Local amendments and project specifications frequently make it mandatory, so check the edition enforced in your area.
Does voltage drop apply to feeders too?: Yes. The informational note to NEC 215.2(A)(1) recommends no more than 3% on the feeder itself, and no more than 5% for the feeder and branch circuit combined. The 3% figure is not only a branch-circuit number.
Why does the calculator ask for one-way length?: The formula already doubles the distance for a single-phase circuit. Enter the distance from the source to the load, not the round trip.

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Cite this page

Use this reference in a report, article, course note or dataset. Calculated examples use this site's engine; reference data and explanatory text have separate sources and review status.

ElecCalcs. “The 3% Voltage Drop Rule, Explained.” Updated October 29, 2026. https://eleccalcs.com/guides/voltage-drop-3-percent-rule

Values reflect NEC 2023 and are pending review. Cite the edition you verified against.

Educational information only — not professional engineering advice. Reference table values are pending review; see the methodology page.